Thousands of protesters took to the National Mall on Sunday for a climate rally that organizers touted as the largest of its kind in U.S. history. The group’s top priority was to urge President Barack Obama to reject the proposed Keystone XL pipeline that would carry oil sands from Canada through the United States.

“When you are in a hole stop digging. Above all, stop the keystone pipeline. The president can do that with the stroke of his pen,” said Bill McKibben, founder of the environmental activist group 350.org.

Although the crowd count could not be independently confirmed, organizers estimated nearly 40,000 people from across the country descended on the nation’s capital to gather near the Washington Monument and then march to the White House for the “Forward on Climate” rally.

Organizers, including the Sierra Club, used the slogan “Forward,” the same one Obama used during his 2012 reelection campaign, to send a message to the White House to follow through on promises to address climate change. The president had addressed the issue throughout his campaign and then again during last Tuesday’s State of the Union.

Sen. Sheldon Whitehouse, D–R.I., said part of the reason for the rally was to “get the fellow in the White House to follow up on the wonderful things he’s said in speeches recently and put a really strong regulatory regime in.”

Obama initially rejected the Keystone XL pipeline in January 2012, saying he wanted more time for an environmental review. A final decision is expected soon. The State Department, under the new leadership of Sec. John Kerry, is currently reviewing the permit application for the pipeline, and White House officials have delayed comment on the president's thinking until after the review is complete.

Pressure has mounted since last month when Republican Nebraska Gov. Dave Heineman sent a letter to the president informing him he now supported a new route the pipeline would take through his state. The Republican governor originially opposed Keystone over concerns that it would disrupt environmentally sensitive areas of his state.

Adding to that pressure is Canada's support of the pipeline that would carry oil from Alberta's oil sands projects to refineries in the Gulf Coast. If the U.S. decides not allow construction, Canada may need to look elsewhere for new energy markets, and the damage could be done to the relationship between the two countries.

For more than a year, Republicans in Congress have attacked Obama for what they view as delaying the project. Conservative argue the 1,000-plus-mile pipeline will provide the U.S. with a reliable energy source and create jobs during a period of slow economic recovery.

But those gathered at the rally today say the pipeline will release more carbon, further heating the Earth and causing more devastating events like droughts and extreme storms.

Other issues the rally focused on were preventing drilling in the Arctic and regulating smokestacks and refineries.

“It’s inspiring. This is the largest rally on climate change in U.S. history,” said Michael Brune, executive director of the Sierra Club. “People have come from almost all over the country, from every state, to really send a strong message to the president that we have his back every time he stands forward on climate change and clean energy, and we want him to use his full executive authority to fight this throughout his whole term.”

And though the protesters took their march to the White House, their message fell on deaf ears, at least for today. President Obama on Sunday was in Florida, where he played golf with Tiger Woods.